The UD pulley is meant for cars that are tracked regularly and will sustain high rpm driving for long periods of time. The underdrive pulley reduces the RPM of the power steering pump, the water pump, and other accessories to lessen wear on them caused by sustained high rpm driving. During normal driving, this will cause accessories to spin at a lower rpm and therefore provide less power to them. Whether the effect is noticeable or not is debatable. However, the tiny bit of a horsepower you may gain by installing an underdrive pulley hardly seems worth the effort.
Again, it is meant for cars that are tracked and is for reduced wear on accessories in that situation, not for hp gain on street cars. Any hp gain would be minute...it is said that up to 5hp is gained at maximum rpm, but that would only be noticeable if you had an accelerometer in your butt. I have yet to see any before and after dyno results, but then I haven't looked either.
Not trying to bust your chops, just the facts from an engineer
Nothing wrong with installing it on your street car...until you are wondering why your AC isn't as cold as you'd like in the sweltering heat or why your car is running hot and dumping antifreeze.
That's just my opinion...and you know what they say about opinions
Personally, I trust the engineers at Porsche. At least, all except for the guy that designed the IMSB
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